The invention relates to a device for blow-forming containers of a thermoplastic material that has a heater unit for temperature treating preforms and at least one blow station equipped with mould supports that can be positioned relative to one another and wherein at least two cavities are located in the area of the blow station whose longitudinal axes have a spacing relative to one another that is larger than a spacing between the preform longitudinal axes of adjacent preforms in the vicinity of the heater unit.
The invention additionally relates to a method for blow-forming containers of a thermoplastic material, wherein preforms are temperature-treated and shaped into containers in the area of at least one blow station and wherein moulds to define the container shape are held by mould supports that can be positioned by support arms, and wherein at least two preforms are simultaneously shaped into containers within each blow station and are positioned such that the preforms assume a positioning relative to one another when entering the blow station that differs from the positioning during the blow forming process.
Devices of this nature are used to shape preforms of a thermoplastic material, for example preforms of PET (polyethylene terephthalate), into containers within a blow-forming machine. Typically, such a blow-forming machine has a heater unit and a blow unit, in which the previously temperature treated preform is expanded to a container by biaxial orientation. The expansion is accomplished with the use of compressed air, which is introduced into the preform to be expanded. The process sequence of such an expansion is explained in DE-OS 4,340,291. The device is also fitted out to remove completed blow-formed containers from the blow unit and to transport them further.
The basic structure of a blow station for forming containers is described in DE-OS 4,212,583. Options for temperature treatment of the preforms are explained in DE-OS 2,352,926.
Within the device for blow forming, the preforms and the blow-formed containers can be transported by means of various handling devices. One proven technique in particular is the use of transport mandrels onto which the preforms are placed.
The preforms can also be handled with other carrying devices, however. For example, the use of grippers for handling preforms is described in FR-OS 2,720 679. An expansion mandrel that can be introduced into a mouth area of the preform for the purpose of holding is explained in WO 95 33 616.
The aforementioned handling of the preforms takes place, on the one hand, as part of the so-called two-stage process, in which the preforms are first manufactured in an injection moulding process, then are stored temporarily, before later being conditioned with respect to their temperature and blow formed into containers. On the other hand, application is also found in the so-called one-stage process, in which the preforms are appropriately temperature treated and then blow formed immediately after their production by injection moulding and adequate hardening.
As regards the blow stations employed, various different embodiments are known. In blow stations that are arranged on rotating transport wheels, one frequently encounters mould supports that swing open in a book-like fashion. In stationary blow stations, which are especially suitable for accommodating multiple cavities for container moulding, plates that typically are arranged parallel to one another are used as mould supports.
For the most part, one container is blow formed at a given time in a blow station. However, especially in the manufacture of small containers, it can be advantageous to simultaneously blow mould two or more preforms into containers in one blow station. Such a method is described in WO-PCT 95/05933 and WO-PCT 96/26826, for example. Simultaneous expansion of multiple preforms in conjunction with rotating blow wheels is already known in the so-called one-stage process and in extrusion blowing, as well. In blow forming machines with stationary moulds, the majority of machines are equipped with blow stations that have multiple cavities.
Different possibilities for feeding a plurality of preforms to a blow station with multiple cavities, and for removing a plurality of blow-formed containers from such blow stations, are described in DE-OS 198 10 238.
The known designs for blow stations with multiple cavities still cannot meet all the requirements placed on production of containers for a high output rate combined with gentle material handling and high mechanical reliability.
The object of the present invention is to design a device of the aforementioned type that supports gentle material handling with high production output.
This object is attained in accordance with the invention in that a positioning element that changes the spacing of the preforms relative to one another is located in the area of the blow station, and in that a spreader element that likewise modifies the spacing of the preforms is also located between at least one heating element and the blow station.
Another object of the present invention is to specify a method of the aforementioned type such that simultaneous transfer of at least two preforms to the blow station is supported.
This object is attained in accordance with the invention in that, between at least one of the heating elements that heat treats the preforms and the blow station, at least one additional change in the spacing of adjacent preforms relative to one another is performed.
The variable-spacing handling of the preforms in the area of the blow station makes it possible to feed the preforms next to one another in the area of the blow station, and to wait until the vicinity of the blow station to define the final positioning and to provide the spacing of the preforms intended for blow-forming. Conversely, after completion of blow-forming and separation of the blow-formed containers from the cavities, definition of spacing of the blow-formed containers can be undertaken in such a way that the optimal spacing conditions required for output are present.
The proposed at least two-step spacing change of the preforms makes it possible to achieve improved kinematics, since the localization of each change in spacing can be adapted to the method of operation of the handling and transfer devices used without it being necessary to predefine a fixed value for a spacing change to be performed in the area of an individual localization. The division of the spacing change among several different locations also makes possible the compact handling of two or more preforms or containers, since the largest spacing, which requires the most handling room, is not established until the preforms are within the blow station.
A compact configuration is supported in that the spreader element is arranged in the vicinity of the heater unit.
Another embodiment consists in that the spreader element is arranged after the heater unit in the direction of preform transport.
A transport motion of the preforms can be carried out with a simultaneous spreading motion in that the spreader element is embodied as a chainlike deflection of support elements for the preforms.
Great flexibility of application can be achieved in that the spreader element is located in the area of a transfer wheel.
High functional reliability can be supported in that a plurality of spreader elements are located in the area of the transfer wheel located between the heater unit and the blow wheel.
A simple basic mechanical structure is provided in that the spreader element has two spreader levers and an actuator.
A mechanical implementation as an articulated lever can be achieved in that the spreader element has positioning levers in addition to the spreader levers, and in that the positioning levers are pivotably connected to the spreader levers as well as to the actuator.
High precision of reproducibility in the execution of actuating motions is achieved by the means that the actuator has a cam roller that can be acted upon by a mechanical cam control system.
The mechanical complexity of the system can be minimized in that a two-step spacing change is performed.
Optimum adaptation of each spacing change to the existing limit conditions of the design can be accomplished in that a three-step spacing change is performed.
Continuous material handling is supported in that the relative spacing of the preforms is increased at every change of spacing.